Ozzy Osbourne may be a rickety, self-described “lunatic.” But as the Black Sabbath frontman proved during the English rockers’ headlining set on the opening night of Lollapalooza Friday, there’s still fire burning deep in his Satan-loving heart. “Fuckin’ A, man! Let me hear you!” the 63-year-old singer, bedecked in his trademark black top and glittering cross, bellowed to the crowd gathered on the North Lawn of Chicago’s Grant Park.

Sabbath’s Lollapalooza set was a momentous occasion: the two-hour long spectacle marked the last of three reunion performances by the hard rock crew, and their only one in the U.S. That original drummer Bill Ward was absent – the result of a contractual dispute – hardly affected the quality of the performance; in fact, Osbourne’s touring drummer, Tommy Clufetos, was an adept, if not impressive substitute.

At the show’s outset, they launched into a quartet of tunes – “Black Sabbath,” “The Wizard,” “Behind the Wall of Sleep” and “N.I.B.” – and it became clear that Sabbath’s star power laid squarely in the hands of guitarist Tony Iommi and bassist Geezer Butler. With Osbourne’s voice noticeably lacking, the well-oiled twosome held things together. Butler, clad in a black leather jacket, remained a stoic presence as he uncorked the brooding bass groove to “War Pigs”; and Iommi, who was introduced by Osbourne as “the true Iron Man,” – he’s currently battling lymphoma – laid into the punishing, timeless riff for the song whose mantra he’s come to represent.

Osbourne was, of course, quite the showman: he introduced the innuendo-packed “Snowblind” by stating that it was a song about one of the band’s “favorite pastimes,” and he must have told the crowd “I can’t fucking hear you” 20 times. (He may not have been lying.)

At the other end of Grant Park, the Black Keys kicked off their fifth appearance at Lolla. It’s been a long road since singer-guitarist Dan Auerbach and drummer Patrick Carney first shred at the Chicago fest in 2005. Now a mainstage staple, the duo attracted an estimated 60,000 fans and got an introduction from Chicago Mayor Rahm Emanuel. They kept things simple, with no guest collaborations or big surprises, apart from a giant disco ball descending down to accompany Auerbach’s soulful falsetto on “Everlasting Light.” The Keys played a handful of “oldies but goodies,” like “Same Old Thing” and “Strange Times,” next to recent El Camino hits “Gold on the Ceiling” and “Lonely Boy.” But the highlight came mid-set. “We’re going to play just the two of us now,” Auerbach told the crowd before busting into “Thickfreakness,” “Your Touch” and the slow-building “Girl Is On My Mind,” sans touring members. It was pure Black Keys: raw, loud and rocking.

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“Who saw us the last time we were at Lollapalooza?” Passion Pit singer Michael Angelakos asked the sprawling crowd gathered for a mainstage set that mixed material from the band’s debut, Manners, and their newest, Gossamer. “Anybody?” The singer had a point: in the three years since Passion Pit’s debut, the band has evolved from side-stage up-and-comers to heavyweights. “The crowds keep getting bigger,” Angelakos told Rolling Stone earlier over lunch at his hotel. “It’s a bit daunting.” Fear however, did not overtake the singer: with his three band members anchoring him, Angelakos scampered about the stage unleashing his whimsically high-pitched vocals on songs new (“Carried Away,” “Take a Walk”) and old (“Moth’s Wings,” “Sleepyhead”).

California DJ-producer Bassnectar was a standout at Lolla’s dance-leaning Perry’s stage, his long hair swinging as he rocked with the music from the beginning to the end. The EDM star blended dubstep into hip-hop rhythms with ease, and his light show was more ambitious than that of his openers, shifting from a starscreen one moment to a solar flare the next, while a rapt audience threw glowsticks over the crowd. Earlier in the day, U.K. duo Nero hit the stage wearing matching black T-shirts and twin reflective sunglasses and delivered set of manic dubstep and electronic music. The young, neon-soaked crowd was loudly appreciative, and the duo were joined by Alana Watson for set highlight (and Number One U.K. hit for nearly year) “Promises.” As rainbow umbrellas bobbed throughout the crowd, fans sang along to the record’s appropriately-themed tribute to intoxication: “Still feel oh so wasted on myself.” Porter Robinson’s style is as influenced by the soundtrack to Dance Dance Revolution as traditional dance music, and it showed during his set. The 22-year-old DJ pumped his fists and waved his hands along to the beats, occasionally even playing air-keyboards as if he were controlling every note of his intense build-ups. The bass twisted as flip-flop-clad fans, their feet dusty from the ballpark dirt, jumped up and down to Daft Punk remixes and Robinson’s originals.

M83’s performance should have been a festival highlight; the Sony stage at dusk gave a view of virtually the entire Chicago skyline, from the Amoco building to South Loop condo – and their svelte, euphoric sound was matched by the cool summer breeze that blew in off the lake. But the volume was much too low for the concertgoers who filled the field, and small talk occasionally overwhelmed the performance. At one point, fans chanted “Turn it up!” at the sound booth. But “Midnight City,” M83’s signature anthem, did re-captivate the audience as the show came to a conclusion.

Emily Haines took to the Bud Light stage on Friday afternoon in leather shorts, yellow shades and a striking multi-colored top that stood out in the sun like shattered stained glass. She bounded across the spacious outdoor stage, where the band’s music translated incredibly well; their guitars carried a visceral, churning energy, particularly in set highlight “Help I’m Alive.” The group jumped directly from track to track with minimal stage banter, concluding with the one-two punch of “Dead Disco” into “Stadium Love.”

Seattle-based folkies the Head and the Heart ignited a mild-mannered shimmy-and-shake party with tasteful helpings of their 2011 eponymous debut – including a rollicking “Cats and Dogs” – just as Dhani Harrison and his electro-nerd project, thenewno2, spazzed out on the intimate BMI stage. The band’s set was heavy on material from their just-released second album, thefearofmissingout, yet Harrison, wearing a black-and-white striped shirt and getting both a guitar and synth workout, seemed supremely confident – especially when executing challenging falsetto runs on “Timezone.”

As the midday sun blazed, it was a pleasure to bask in the mellow moods of the early acts. U.K. singer-songwriter Michael Kiwanuka unfurled a set of slow-rolling, blues-informed numbers from his debut,Home Again. Offstage, the Londoner told Rolling Stone that while he was bummed to be away during the Olympics, Lollapalooza wasn’t a bad alternative. “When you get to play Lollapalooza in the sun, it’s a pretty cool substitute,” he said. Brooklyn’s Sharon Van Etten kept the tender vibes intact with the dreamlike haze of “I’m Wrong” and “Save Yourself.” “My Nineties dreams are coming true today,” the singer told the crowd. Backstage, she told Rolling Stone she had always wanted to come to Lollapalooza as a teenager. Strict parenting was her only obstacle. “I wasn’t allowed!” she said, laughing.